The invention relates to a brake disc for a disc brake, having an outer friction path on one side and an inner friction path on the other side of the brake disc and having air-swept hollows (recesses) between the friction paths for cooling the brake disc, as well as to a disc brake, in which such a brake disc is fitted, the brake disc therefore being a so-called ventilated brake disc.
Such brake discs are known from DE 10 2007 010 487 A1.
In physical terms, the braking operation in a motor vehicle means that its kinetic energy is converted substantially into heat. This heat is generated in the brake and has to be dissipated during the actual braking operation for the most part into the environment. During a total braking operation from higher speeds thermal outputs of 500 kW may arise. If a motor vehicle is braked over an extended time, for example when travelling down from a mountain pass, then thermal outputs in the region of 5 kW typically arise.
The conversion of the kinetic vehicle energy into heat is effected at the friction surfaces of the brake. In the case of a disc brake, these are the friction surfaces between the brake linings and the brake disc.
Heat is removed from the brake disc by radiation, convection and thermal conduction. Nevertheless considerable heating of the brake disc occurs during more extreme and/or extended braking operations. The heating of the brake disc generally does not occur homogeneously over the entire brake disc. Rather, temperature gradients arise and may lead to deformation of the brake disc, in particular to the known so-called umbrella effect, i.e. an arching of the brake disc. Such deformations are highly undesirable. In particular, they may lead to so-called “brake judder”.
A further problem that repeatedly has to be solved with disc brakes is so-called brake squeal, which may occur i.e. also because various vibration effects may arise through reciprocal action (coupling) in the brake. For example, there are couplings of the two mechanical sub-systems “caliper” and “brake disc” particularly if their resonant frequencies lie close to one another. This may lead at specific brake forces to a resonant coupling and hence to extremely undesirable squealing noises.
In the initially mentioned DE 10 2007 010 487 A1 an attempt is made to combat the described problems by disposing a compensating ring on a radially inner edge of the friction disc (brake disc). The intention here is to shift the natural frequencies and in particular the axial bending modes towards higher frequencies, thereby making a vibration coupling with other sub-systems of the brake difficult. Radial forces, which act as a result of thermal expansion upon the brake disc, are also to be absorbed in the region of the compensating ring and hence an umbrella effect on the brake disc is to be avoided.
From U.S. Pat. No. 2,629,464 a ventilated brake disc is known, in which the connection of the disc pot to the disc rim is reduced in cross section and has a curvature, a so-called swan neck. The swan neck extends in axial direction (in relation to the axis of rotation of the disc) to a point under the disc rim. The intention here is to adapt the connection between disc pot and disc rim to the expansion and contraction of the disc rim as a result of thermal effects without overstressing the material, in order therefore to avoid crack formation in particular.
From U.S. Pat. No. 2,800,982 a ventilated brake disc is known, in which between radially extending thicker ribs thinner ribs are disposed. The thicker ribs form the supporting connection between inner friction path and outer friction path, while the thinner ribs mainly as a result of surface enlargement are used to dissipate heat. According to a form of construction the thinner ribs are provided radially at the outside and radially at the inside with V-shaped cutouts. In this way, more heat is to be removed from the central region of the friction paths and the overall result is to be that the temperature in the friction rim remains as homogeneous as possible in order to reduce thermal gradients and hence thermal stresses.
From FR 1,309,093 a solid brake disc is known, in which the connection of the brake disc pot is effected approximately centrally in relation to the two friction paths. To combat the umbrella effect, a groove is cut into the region of the connection in such a way that the inner friction path and the outer friction path have approximately the same radial width. By tuning the dimensions of the groove the thermal conduction from the friction paths into the disc pot is to be controlled in such a way that a reduction of the umbrella effect is achieved.
DE 1 283 611 discloses a ventilated brake disc, in which the friction path, which verges directly into the connection to the brake disc pot and from which therefore more heat is removed by thermal conduction than from the other friction path, has a lower wall thickness than the other friction path. The intention here is to achieve a more homogeneous temperature distribution, thereby preventing stresses and cracks in the region of the friction paths.
DE 1 950 178 discloses a ventilated brake disc, in which the friction path that verges directly into the connection to the brake disc pot has a greater wall thickness than the other friction path. The intention here is that cracks due to thermal stress may occur only after a very long operating period and very extraordinary stresses. The reason for this that is cited there is that the strengthened wall of the friction path, which in the operating state is situated in the wheel disc and from which the air heated during braking may be only incompletely dissipated, is better able to withstand the high specific heat loads and ensures a more uniform heat distribution in the disc body. Here, in order to prevent extreme warping of the brake disc, a groove is provided also in the region of the connection of the outer friction path to the brake disc pot.
DE 2 128 594 discloses a fully lined disc brake having a solid, i.e. non-ventilated, brake disc, in which the connection of the brake disc pot is effected approximately centrally in relation to the two friction paths (by centrally is meant in axial direction between the friction paths). Between the two friction paths a deep V-shaped groove is cut so that upon actuation of the brake the friction paths spring relative to one another. In the non-actuated state of the brake the friction paths extend, not plane-parallel to one another, but in the shape of a double cone, such that their width at the outside diameter is greater by the extent of the compliance than at their inside diameter. The intention here is to achieve a full surface contact of the friction surfaces of the brake linings and the friction paths in order to achieve a uniform distribution of the contact pressure and hence also of the friction forces and the heating. The uniform thermal load is also to lead to uniform wear of the brake linings. In order to reduce the heat flow on the side of the connection to the pot, the connection is interrupted by a plurality of openings and the wall thickness is reduced.
U.S. Pat. No. 3,892,297 and GB 2 001 716 also deal with the problem of preventing thermal gradients in the brake disc and for this purpose provide different wall thicknesses for the friction paths.
DE 3 026 354 describes a ventilated brake disc having thermal correction masses in the region of the ventilating channels. The outer friction path, which, there, is connected directly to the disc pot, is enlarged in such a way that the mass of the outer friction path including the said thermal correction mass corresponds approximately to the mass resulting from the sum of the masses of brake disc pot, connection of the brake disc pot and transition to the outer friction path. In this way, with regard to the removal of the heat from the outer friction path into the disc pot, which acts as a heat sink, and the associated temperature differences between the radially outer zone and the radially inner zone of the outer friction path a correction is provided to the effect that the heat of the outer friction path flows into the thermal correction mass and hence the temperature differences within the outer friction path are reduced.
DE 3 231 175 discloses i.e. a ventilated brake disc having a V-shaped indentation between the two friction paths. In order upon brake actuation to keep the two friction paths a defined axial distance apart, supporting webs or flat annular bodies are disposed between the friction paths. In this way, the radially outer region of the friction paths is to be heated to a greater extent than the radially inner region, with the result that the outer region expands to a greater extent than the inner region. The intention here is to generate a tensile stress that ensures a tautening of the friction paths in radial direction and hence prevents warping of the brake disc.
WO 2004/102029 A1, and corresponding U.S. Pat. No. 7,703,583 both of which are incorporated by reference herein in entirety, discloses a ventilated brake disc having plates, which form the friction paths and are connected to one another by supporting elements, wherein relatively large free spaces between supporting elements are provided for intensifying the air flow.
U.S. Pat. No. 5,460,249 describes a ventilated brake disc, in which for preventing thermal deformation ribs are provided between the plates forming the friction paths and extend beyond the inner edge of the brake disc.
US 2005/0269173 A1 describes a ventilated brake disc, in which for preventing an outward bending of the brake disc during brake operation a special arrangement of supports is provided between the plates forming the brake disc.